The present invention relates to a scale inhibitor for water and aqueous systems, and more particularly to an agent made of oxidized polysaccharide that inhibits the formation of scales.
Recent industrial growth and development has resulted in the utilization of vast quantities of water, to the point where a curtailment in the use of cooling water, demand for which is especialy large, has become necessary and desirable. Because of this, the recirculating of cooling water has become unavoidable in office buildings and factories, and the number of water systems practicing high-concentration operation, which keeps the amount of blowdown to an absolute minimum and keeps the high-concentration of salts in recirculating water, is increasing. Ions normally present in water, such as calcium ions, magnesium ions, carbonate and bicarbonate ions, sulfate ions, and silicate ions, tend to build up in water systems such as these. This results in the deposition of sparingly soluble salts on the heat exchanger, leading to scale formation, which reduces the heat exchange capability and causes clogging and other problems. Such scaling problems are not restricted only to cooling water systems, but occur also in boiler water systems, and in water and aqueous systems in desalination plants, geothermal plants, iron mill stack gas and dust collector effluent systems, incinerators, and pulp mill digester liquor and waste liquor evaporators.
Polyphosphates such as sodium tripolyphosphate and sodium hexametaphosphate have hitherto been used to prevent such scaling. However, because these are phosphorus-containing materials, when discarded as waste, they cause the eutrophication of sea areas, rivers, and lakes. Yet another problems is that these compounds are hydrolyzed in water to form orthophosphoric acid, which results, paradoxically, in the formation of calcium phosphate scales. For these and other reasons, phosphonic acids such as monoaminotrialkylphosphonic acid, hydroxyalkylidenediphosphonic acid, monophosphonopolycarboxylic acid, and synthetic polyelectrolytes such as polyacrylic acid and polymaleic anhydride are being used of late. Yet, although the resistance to hydrolysis in water of these phosphonic acids, for example, is greater than scale inhibitors previously used, the fact remains that, as phosphorus substances, these cause eutrophication. Another drawback is that, of the synthetic polyelectrolytes, polyacrylic acid has a relatively limited effectiveness, and polymaleic anhydride is expensive.
Moreover, in cases containing large amounts of silicate ion, the effectiveness of these chemicals is greatly reduced or virtually suppressed altogether.
We conducted studies aimed at coming up with safe new scale inhibitors capable of replacing the agents currently in use, but containing no phosphorus. In the course of our investigations, we discovered that oxidized polysaccharides containing carboxyl group-bearing units, which obtained by the oxidative cleavage of natural polysaccharides having 6-membered monosaccharide rings, exhibit highly effective scale inhibitive properties.
This led ultimately to the present invention, which relates to a scale inhibitor comprising an oxidized polysaccharide containing, as the essential structural units, carboxyl group-bearing units of the formula: ##STR2## where A is CH.sub.2 OR, R is H, (CH.sub.2).sub.n --COOM, or COOM, M is H, an alkali metal ion, or an ammonium group, and n is an integer from 1 to 3, obtained by the oxidative cleavage of polysaccharides containing 6-membered monosaccharide rings.
The polysaccharides containing 6-membered rings used as the starting material in the formation of the scaling inhibitor of the present invention is a polysaccharide having vicinal hydroxyl groups. Examples include starches such as corn, potato, rice, arrowroot, and tapioca, dextrin, cellulose, sawdust, polyuronic acids such as alginic acid, agar, and natural rubbers such as gum arabic. Substances such as starch, dextrin, and cellulose that have undergone partial chemical modification, such as hydrolysis, esterification, etherification, or carboxylation, may also be used. Of these, the use of starches, cellulose, or their carboxymethyl ethers are preferred because of their availability, price, reactivity, and other factors. The best choice of all is a starch.
Techniques that may be used to oxidize these polysaccharides include methods that employ well-known oxidizing agents, such as halogens, hypochlorites and other halides, chromic acid, permanganate, and the hydrogen peroxide-iron salts, capable of cleaving the glucose groups and introducing carboxyl groups; and methods involving treatment with well-known oxidizing agents, such as periodic acid, lead tetraacetate, silver salts-persulfuric acid, copper chloride, copper hydroxide, and thallium salts, capable of cleaving glucose groups and introducing aldehyde groups, followed by treatment with well-known chemicals capable of converting the aldehyde groups into carboxyl groups, such as a halogen, hydrogen peroxide, or nitric acid. In terms of reactivity and cost, methods consisting of one-step oxidation using sodium hypochlorite, halogens such as chlorine or halides are especially favorable here.
The polysaccharide and oxidizing agent may be reacted by any well-known method employed for the particular oxidizing agent. For example, when sodium hypochlorite or a halogen such as chlorine is used as the oxidizing agent, the desired product can readily be obtained by reaction for one to several hours at room temperature or under heating at a pH maintained within the range of 7-10. Best results can be obtained at a pH 8-9.
There is no need for the polysaccharide starting material to be oxidized until fully converted to a structure containing the carboxyl group-bearing units depicted in the above formula. Substances such as the oxidized starch sold commercially as paste for use in the papermaking industry and elsewhere are also noted to have a scale inhibiting effect, although these are oxidized only to a limited degree, with just several percent of the glucose units within the molecule containing carboxyl group-bearing units. However, a fairly large amount of this type of oxidized polysaccharide is required to provide an adequate scale-inhibiting effect. In addition, prior to use, it must be heated and dissolved, further detracting from its practicability. Therefore, in order for this substance to exhibit good properties as a scale inhibitor, at least 10% of the glucose units within the molecule should contain carboxyl group-bearing units. And for maximum performance and properties as a scale inhibitor, at least 40% should contain carboxyl group-bearing units. Oxidized polysaccharide that has been modified such that at least 40% of the glucose units within the molecule contain carboxyl group-bearing units becomes an aqueous solution, are ideally suited for addition as a scale inhibitor to water.
The amount of oxidizing agent added to the polysaccharide starting material need only be that amount required to oxidize the polysaccharide to the point where the polysaccharide molecule contains the carboxyl group-bearing units in the above proportion. In general, the greater the amount of oxidizing agent added, the higher the ratio of the carboxyl group-bearing units represented above in the formula. For example, when using sodium hypochlorite, to obtain a content of carboxyl group-bearing units of about 5% based on the starting material, it is necessary merely to add an amount corresponding to 0.5 equivalent of effective chlorine per equivalent of glucose units in the polysaccharide. And to obtain a product containing at least 40% of carboxyl group-bearing units, the addition of at least 2 equivalents is needed.
The carboxyl group-bearing units in the oxidized polysaccharide serving as the scale inhibitor according to the present invention may be arranged in any order within the molecule. Furthermore, in addition to the carboxyl group-bearing units, unreacted glucose units and cleaved aldehyde group-bearing units may also be present in the oxidized polysaccharide.
It is not clear in what form these units are present in the oxidized polysaccharide; i.e., whether they exist as mixtures of various polysaccharide molecules, or as regular or irregular copolymers. In addition, the molecular weight of the oxidized polysaccharide varies depending on the molecular weight of the polysaccharide as starting material, the type of oxidizing agent, and the reaction conditions. But, what is of most crucial importance in the present invention is that the product contains the carboxyl group-bearing units represented by the formula given above.
The oxidized polysaccharides obtained are generally low molecular weight polymers having a smaller degree of polymerization than the starting material. This can be determined, for example, by iodine color reactions or viscosity measurements with starch as the starting material.
The oxidized polysaccharide thus obtained may be used directly in the form of the reaction product, but it is also possible to neutralize the product, reduce the moisture content through concentration, or obtain a powder by precipitation and recovery from an organic solvent such as an alcohol or acetone.
The amount of the oxidized polysaccharide added to water or aqueous systems as the scale inhibitor in the present invention varies with the condition or degree of scaling and the content of carboxyl group-bearing units, but in general lies within a range of 1 to 1000 ppm, and preferably 1 to 100 ppm. For example, when the ratio of carboxyl group-bearing units is 40% or more, the addition of 1-20 ppm is sufficient. In such cases, the addition of more than 20 ppm does not provide a significant improvement in effectiveness.
Addition to a water or aqueous system may be continuous or intermittent, but in either case, it is desirable that control be applied to the system such that the above effective concentration is maintained therein.
Known polymers or copolymers having carboxyl substituted ethylenic hydrocarbons as monomeric constituents, e.g., the polyacrylic acid or hydrolyzed polymaleic anhydride or the water-soluble salts, may be used in combination with the scale inhibitor according to the present invention. The joint use of such polymers or copolymers can be very effective when the water or aqueous system being treated is characterized by certain special conditions, especially high temperature.
These polymers or copolymers may be, for example, ethylenic hydrocarbon polymers having carboxylic groups such as acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, fumaric anhydride, itaconic acid, and crotonic acid; copolymers of these monomers or of these monomers and other polymeric vinyl monomers, e.g., acrylonitrile, acrylamide, methacrylamide, vinyl acetate, styrene, styrene derivatives, ethylene, n-butylene, and isobutylene; or the hydrolysates or water-soluble salts of these polymers or copolymers. Copolymers with other polymeric vinyl monomers that are generally used have a mole ratio of ethylenic hydrocarbons having the above carboxyl groups to other polymeric vinyl monomers of from 1:1 to 100:1. These may be known polymers or copolymers, such as those cited in Japanese Patent Publication Nos. 44-11791 and 53-20475. Of these, in terms of availability, price, and other factors, it is most desirable to use polyacrylic acid or its water-soluble salts, or hydrolyzed polymaleic anhydride or its water-soluble salts. These are generally used at molecular weights of not more than 100,000 for optimal dispersability and flocculating ability, but normally at not more than 50,000, and for best results, at about 500-15,000.
When these polymers or copolymers are used together with the scale inhibitor according to the present invention, the mixing ratio by weight of scale inhibitor to polymer or copolymer is from 9:1 to 1:3, and preferably from 5:1 to 1:1.
As for the method of mixture, it is possible to either add a premixed formulation to the water or aqueous system, or to add the components separately and have them mix within the system. The amount added depends on the condition or degree of scaling, the water or aqueous system conditions, and other factors. In general, however, the concentration within the water or aqueous system should lie in the range 0.1-100 rpm. At too low concentrations, not enough of an anti-scaling effect is produced, whereas use in quantities exceeding this range is costly without producing significant improvements in results. A quantity that gives a concentration of 1-10 ppm is normally adequate.
The use in combination with polymers or copolymers having carboxyl substituted ethylenic hydrocarbons as monomer components permits outstanding results to be obtained despite the addition of relatively small amounts, even in water or aqueous systems in which fully adequate effects cannot be obtained by the use of one or the other of these components alone. This is of great significance for industrial applications.
Not only is the scale inhibitor according to the present invention superior in terms of cost and the requisite production equipment on account of its use of inexpensive polysaccharides and oxidizing agents and the ease of performing the synthesis reaction, but is also extremely effective against a broad variety of scales, even exhibiting excellent deposition inhibiting effects against silica-type scales, which have been almost entirely resistant to previous chemicals.
The present invention is further illustrated through the following preparations and examples.